1. Field of the Invention
The present invention relates to a position detection apparatus and an exposure apparatus.
2. Description of the Related Art
Background arts of a position detection apparatus and exposure apparatus will be explained by taking a semiconductor exposure apparatus which requires a strict measurement of the surface position as an example.
Photolithography (exposure) is often used to manufacture a micropatterned semiconductor device such as a semiconductor memory or logic circuit or a liquid crystal display device on a substrate. In this case, an exposure apparatus (e.g., a projection exposure apparatus) is used to project and transfer a circuit pattern drawn on a mask (e.g., a reticle) onto a substrate (e.g., a wafer) via a projection optical system.
Along with an increase in the degree of integration of semiconductor devices, the exposure apparatus is required to project and transfer the circuit pattern of a mask onto a substrate by exposure with a higher resolving power. A minimum dimension (resolution) with which the exposure apparatus can transfer is proportional to the wavelength of exposure light and is inversely proportional to the numerical aperture (NA) of the projection optical system.
That is, the shorter the wavelength, the better the resolving power. In view of this, the recent light sources often use a KrF excimer laser (wavelength: about 248 nm) and an ArF excimer laser (wavelength: about 193 nm) which have wavelengths shorter than those of superhigh pressure mercury lamps (the g-line (wavelength: about 436 nm) and the i-line (wavelength: about 365 nm)).
The higher the numerical aperture (NA), the better the resolving power. In view of this, a study for the practical application of immersion exposure is also in progress.
Another demand has arisen for further widening the exposure region. To meet this demand, there is available an exposure region widening technique of forming an exposure region having a rectangular slit shape (exposure slit region), and relatively scanning a mask and substrate at high speed. An exposure apparatus (also called a “scanner”) of a step-and-scan scheme using this technique can expose a wide area with a higher accuracy than with an exposure apparatus (also called a “stepper”) of a step-and-repeat scheme which reduces a roughly square exposure region and performs cell projection for it.
During exposure, this scanner causes a focus detection system to irradiate the exposure region on the substrate with measurement light to measure the surface position of the substrate in the optical axis direction of the projection optical system. Based on the detection result obtained by the focus detection system, the scanner exposes the substrate while aligning its position in the optical axis direction with the imaging plane of the pattern of the mask.
Along with the shortening of exposure light and an increase in the NA of a projection optical system, the depth of focus is decreasing extremely. Under the circumstances, an accuracy of focusing the surface position of an exposure target substrate on the imaging plane of the pattern of a mask, that is, focusing accuracy is also increasingly becoming stricter. Particularly, this is because a measurement error of the focus detection system is readily generated due to the thickness nonuniformity of a resist applied to the substrate or depending on the pattern on the substrate.
More specifically, the thickness nonuniformity of the resist readily occurs near a peripheral circuit pattern or scribe line, resulting in an increase in the step on the resist surface. When this occurs, the tilt angle of the resist surface increases locally (a local tilt occurs) even if the substrate has a flat surface, so that the reflection angle of reflected light detected by the focus detection system with respect to the substrate often locally, largely fluctuates. In some cases, the focus detection system may not exactly measure the surface position of the substrate.
A high-reflectance area (e.g., a dense pattern area) and a low-reflectance area (e.g., a sparse pattern area) may coexist on the substrate surface. The reflectance of the substrate to reflected light detected by the focus detection system often largely fluctuates locally even if the substrate has a flat surface. In some cases, the focus detection system may not exactly measure the surface position of the substrate.
For example, as shown in FIG. 20, consider a case in which a high-reflectance area (an area indicated by a white portion) HA and a low-reflectance area (an area indicated by a hatched portion) LA coexist on a substrate SB. A measurement area MM onto which a slit image is projected by irradiation with measurement light has a side in the longitudinal direction (β′ direction), which is tilted with respect to the boundary between the high-reflectance area HA and the low-reflectance area LA by an angle A. The measurement direction is almost perpendicular to the β′ direction, that is, the α′ direction.
FIG. 21 shows the reflected light intensity distributions in three sections perpendicular to the β′ direction, that is, sections AA′, BB′, and CC′. As shown in FIG. 20, the sections AA′ and CC′ have uniform reflectances, respectively. As shown in FIG. 21, the reflected light intensity distributions in the sections AA′ and CC′ have good symmetries. In contrast, as shown in FIG. 20, the section BB′ has different reflectances. As shown in FIG. 21, the reflected light intensity distribution in the section BB′ has a poor symmetry. In this manner, the reflectance of the substrate to reflected light detected by the focus detection system often locally, largely fluctuates even if the substrate has a flat surface. That is, the reflected light intensity distribution often has a poor symmetry in a local area (in the vicinity of the boundary between the high-reflectance area HA and the low-reflectance area LA). Consequently, the measurement value obtained by the focus detection system, that is, the barycentric position of the reflected light intensity distribution locally shifts from the actual surface position of the substrate. In some cases, the focus detection system may not exactly measure the surface position of the substrate.
To solve this problem, Japanese Patent Laid-Open No. 11-16827 discloses a technique of arranging light projecting optical systems and light receiving optical systems of two surface information detection systems (focus detection systems) at positions which face each other on the opposite side of the normal to the upper surface of the substrate. The light projecting optical systems of the two surface information detection systems direct light beams such that they enter the substrate from two different directions on a plane including the normal to the upper surface of the substrate. Upon being reflected by the substrate, the light beams are directed in two different directions on the plane including the normal to the upper surface of the substrate, and detected by the light receiving systems of the two surface information detection systems. As disclosed in Japanese Patent Laid-Open No. 11-16827, the two surface information detection systems measure the surface position of the substrate based on the detection results obtained by the light receiving systems. With this operation, errors of the measurement values obtained by the focus detection systems, which are attributed to, for example, steps, are reduced to some extent.
However, the technique disclosed in Japanese Patent Laid-Open No. 11-16827 separately arranges the light projecting optical systems of the two surface information detection systems (focus detection systems), and also separately arranges their light receiving optical systems. This makes it difficult to ensure almost the same characteristics of the light projecting optical systems and light receiving optical systems between the two surface information detection systems. It is also difficult to allow the two focus detection systems to project slit images having nearly the same shape to almost the same position on the substrate. Even when the light beams are directed such that they enter the surface of the substrate from two different directions, it is impossible to sufficiently reduce errors of the measurement values obtained by the focus detection systems, which are attributed to, for example, steps. In some cases, the focus detection systems cannot accurately measure the surface position of the substrate.